Sensory Receptors

 

Sensation (irritability) – An awareness of changing conditions in the environment.  A sensation depends on the following components:

 

  1. Stimulus – a change in one or more conditions, e.g., light, sound, temperature.  The stimulus can arise from a change in the external or internal environment (inside the body).

 

  1.  Receptor – a cell, tissue or organ designed to respond to one type of environmental change.

 

Receptor Category

Receptor

Stimulus

 

 

 

Mechanoreceptors

Pacinian corpuscle

Pressure

Meissner’s corpuscle

Touch

Merkel cell

Light touch

Cochlea (organ of Corti)

Sound waves

Semicircular canals

Positional changes – balance

Muscle spindles

Proprioception

Golgi tendon organ

Proprioception

 

 

 

 

Chemoreceptors

Taste buds

Chemicals in saliva

Olfactory tract

Chemicals in inhaled air

Carotid body

Oxygen in blood

Nociceptors

Pain due to trauma

 

 

 

 

Thermoreceptors

Cold receptors in epidermis

Cool temperatures (10 to 30 C)

Warm receptors in dermis

Warm temperatures (32 to 48 C)

Nociceptors

Very cold or very hot temp.

 

 

 

Photoreceptor

Retina

Light

 

When the stimulus is received by the receptor cell, a generator potential is produced.  The strength of the generator potential is directly proportional to the magnitude of the stimulus.

 

  1. Conduction – Sensory messages must be conducted from the receptor to the CNS.  These sensory impulses are carried by nerves of the peripheral nervous system.  The frequency of nerve impulses in the sensory neuron is determined by the strength of the generator potential produced in the receptor cell.

 

  1. Interpretation – The sensory messages are interpreted in various regions of the cerebral cortex.  For example, nerve messages from the retina of the eye are made sense of in the visual cortex of the occipital lobe of the cerebrum.  For touch, pressure and pain the interpretive center is in the parietal lobe.  Auditory signals are interpreted in the temporal lobe.

 

I.  The Eye - The eye is our only receptor that is stimulated by light.  The eye ball has three layers or coats:

 

A.  The Sclera or Outer Coat

1.  Composed of the tough sclera.  This layer preserves the shape of the eyeball and protects delicate inner layers.

 

2.  The front of this coat consists of the transparent cornea which permits light to enter the eye.  A cloudiness or cataract may develop in the cornea.  This will necessitate a corneal transplant.

 

3.  Extrinsic muscles are attached to the sclera:

      a. Superior rectus – turns the eye upwards

           

b.      Inferior rectus – turns the eye downwards

 

c.       Lateral rectus – turns the eye outwards

 

d.      Medial rectus – turns the eye inwards towards the nose

 

e.       Superior oblique – turns the eye down and outwards

 

f.        Inferior oblique – turns eye up and outwards

 

B.  The Anterior Compartment

Inside, the eye is divided into the anterior and the posterior compartments.

 

1.      The anterior compartment is found between the cornea and the lens.  It is filled with a watery aqueous humor. 

 

2.      Aqueous humor is filtered out of the blood by the ciliary body, a structure located on the posterior surface of the iris (in the posterior portion of the anterior chamber).  The intraocular pressure of this fluid should be about 15 mm Hg. 

 

3.      Aqueous humor is absorbed back into the blood by the canal of Schlemm which is found on the front of the iris.  A build up in pressure of this fluid can produce glaucoma, a condition which can lead to blindness.

 

4.      The iris is a circular sheet with an opening in the center called the pupil.  This opening is just in front of the lens.  The iris contains two types of smooth muscle fibers:

a.       Radial muscle which attaches to the edge of the papillary opening like the spokes of a wheel.  When this muscle is stimulated by sympathetic fibers of the oculomotor nerve, the pupil gets larger.

b.      Circular muscle which forms concentric circles off smooth muscle within the iris.  When this muscle is stimulated by parasympathetic fibers of the oculomotor nerve, the pupil gets smaller.

            

  1. The lens is a transparent, biconvex structure located just behind the iris.  A  cataract can also develop in the lens requiring lens replacement.  A circular ring of smooth muscle, the ciliary muscle, is attached to the lens by suspensory ligaments. 
    1. The contraction of the ciliary muscle is produced by the oculomotor nerve.  This allows the lens to become thicker in the middle permitting vision of things up close. 
    2. When the ciliary muscle relaxes, the lens becomes thin.  This permits us to focus on things at a distance.  This change in the shape of the lens is called accommodation. 
    3. As we age, the lens loses its flexibility and we are unable to have good visual acuity (sharpness of vision) for near and possible far items.  Correctable lens are needed.  This condition is called Presbyopia.

 

  1. Posterior Chamber – The interior of the eye between the lens and the retina is called the posterior chamber.

 

    1. This space is filled with a transparent jelly-like substance, the vitreous humor, that helps to hold the retina in place and give a nice round shape to the eyeball.

 

    1. The back wall of the eye is formed from three layers of tissue:

a.       The sclera is the outermost coat.  It is a tough, opaque layer.

b.      Inside the sclera is the choroid coat.  This is a heavily melanized layer.  It functions to absorb extraneous light in the back of the eye.

c.       The retina comprises the innermost layer of the eye.  It contains the photoreceptors; rod cells for black and white vision and cone cells for color vision.  Other cells in the retina synapse with the rods and cones.  Finally, sensory axons carry the nerve messages of vision through the optic disc in the back wall of the eye into the optic nerve.  The optic disc has no rods or cones and is called the blind spot.

d.      A very small depression near the optic disc, the fovea centralis, contains the densest concentration of cone cells.  It is responsible for our sharpest vision.  Macula degeneration is a disease involving a gradual deterioration of the cells in the fovea resulting in blindness.

 

 

 

 

 

Common Eye Tests

 

  1. Visual Acuity – A standard eye exam that tests our visual sharpness using the Snellen chart.  If you can read the line of letters with one eye which are designated to be read at 20 feet, by an individual with normal vision, then you have 20/20 vision in that eye.

 

  1. Astigmatism – is the result of an unequal curvature of either the cornea or the lens or both.  This condition prevents focusing on all parts of chart showing converging lines.

 

  1. Color blindness – is a defect in which certain colors of light are unable to stimulate the cone cells.  The most common type of deficit is red-green color blindness

 

 

II.  The Ear – contains a receptor for sound stimuli, the Organ of Corti which is located in the Cochlea.  There are also receptors for balance and equilibrium, the Vestibular apparatus.  The ear is divided into the external ear, the middle ear and the inner ear.

  1. The external ear includes the pinna, an exterior flap of skin and cartilage, and the auditory canal that leads inward through the temporal bone to the tympanic membrane or ear drum.

 

  1. The middle ear is a small cavity in the temporal bone that is connected to the throat or pharynx by the Eustacian tube.  The Eustacian tube helps to balance air pressure on either side of the ear drum.  The middle ear houses three small bones, the malleus (hammer), the incus (anvil) and the stapes (stirrup).  These bones vibrate along with the ear drum as sound waves reach the ear.  The stapes is connected to the oval window of the inner ear.

 

  1. The inner ear consists of interconnecting fluid-filled tubes embedded in the temporal bone.  There are three divisions of the inner ear:

 

    1. The cochlea is a bony structure that is coiled like a snail shell.  It houses the receptor for sound waves called the Organ of Corti.  The receptor cells of the Organ of Corti rest on the basilar membrane which forms the floor of the Organ of Corti.  It is the upward deflections of the basilar membrane due to sound produced pressure changes in the cochlea that stimulates the receptor cells and generates nerve messages through the cochlear nerve.  The cochlear nerve forms one half of the Vestibulocochlear nerve (cranial nerve VIII).  Nerve messages through the cochlear nerve are carried to the auditory cortex of the temporal lobe for interpretation.

 

    1. The vestibule is the enlarged portion at the base of the cochlea.  The oval window of the vestibule is attached to the stirrup bone.  The flexible round window of the vestibule absorbs or dampens the pressure changes within the cochlea.  This improves the clarity of our hearing.  The vestibule also contains receptors for static equilibrium that inform the brain of the position of the head

 

    1. The semicircular canals contain receptors for dynamic equilibrium which informs the brain when the head is turned or when the entire body is rotated.